The present invention relates to a process for preparing a water-absorbent resin, and more particularly to a process for preparing a porous and water-absorbent resin which is suitable for use as a water-absorbent material, a water-retaining material, and the like.
In recent years, so-called water-absorbent resins, which are high in absorbancy to an aqueous liquid, are insoluble in water and are capable of swelling with a large amount of water, have been developed, and they have been widely used for various applications.
An aqueous solution polymerization or a reversed phase suspension polymerization has hitherto been applied to the preparation of such a water-absorbent resin.
Generally, although the aqueous solution polymerization system is forcedly heated at the time when polymerization begins, the aqueous solution is heated by the heat of polymerization during polymerization which makes it difficult to control the polymerization temperature.
Also, in reversed phase suspension polymerization, in general, the dispersion medium is boiled, whereby the heat of reaction is removed from the polymerization system. So, it is easy to control the polymerization temperature, therefore, the reversed phase suspension polymerization is remarkably practical in industrial scale.
It is required to obtain a water-absorbent resin having both a high rate of water-absorption and a high water absorbency. However, the products obtained by reversed phase suspension polymerization do not necessarily possess the desirable properties mentioned above. It has been attempted to make the water-absorbent resin particles porous to improve the physical properties of the water-absorbent resin. For example, in a water-in-oil suspension polymerization wherein an aqueous solution of an ethylenically unsaturated monomer is suspended or dispersed in a hydrophobic medium which is inert to polymerization, polymerization is conducted using a specific dispersing agent is used (Japanese Unexamined Patent Publication No. 60-243106), or a mixture of two kinds of dispersing agents is used (Japanese Unexamined Patent Publication No. 61-97301), whereby a porous and water-absorbent resin is obtained.
According to these methods, however, it cannot be so much expected to improve the rate of water-absorption and water absorbency of the water-absorbent resin, because only the surface of the resin particles are made porous.
It is an object of the present invention to provide a process for preparing a water-absorbent resin having an adequate water absorbency and an improved water absorption rate easily and efficiently as an industrial scale.
This and other objects of the present invention will become apparent from the description hereinafter.